The potential use of brominated polystyrene to flame retard plastic components for use in household appliances, chemical or foodstuff containers, machine components and the like has become well-known. Brominated polystyrenes are typically obtained through the Lewis acid catalyzed aromatic bromination of polystyrene. The styrenic polymers to be brominated are produced by any of the common polymerization techniques. These methods commonly employ ionic or free radical initiated polymerization of styrenic monomers with or without a solvent. Bromination of the polystyrene is typically achieved using a metal halide catalyst such as antimony trichloride and bromine or bromine chloride as the bromination reagent. An organic halocarbon solvent such as dichloroethane is also used in the bromination reaction.
Alternatively, brominated polystyrene may be produced using bromine as the reaction solvent, bromine chloride as the bromination reagent and an anhydrous metal halide catalyst. This process, which is the subject of co-pending U.S. patent application Ser. No. 007,778, now abandoned, permits controlled bromination of the polystyrene. At the same time, however, it affords a product with higher levels of occluded free bromine and ionics as well as higher levels of contained aliphatic halogen. The aliphatic bromine impurities have been found to result in a brominated polymer of lower thermal stability when subjected to formulation conditions resulting in discoloration of the formulation and corrosivity to the processing equipment. Ionics may degrade formulations in respect to their ultimate electrical properties. They may also result in corrosion of processing equipment or in the corrosion of metallic parts in their end-use applications.
Previously known processes for purifying the brominated polystyrene have proven to be both costly and ineffective. In Diebel et al., U.S. Pat. No. 4,200,703, a purification procedure is disclosed for ionic halogen removal from a brominated polystyrene. In accordance with the procedure disclosed there, the ionic halogen or salts are removed through multiple aqueous washes of a brominated polystyrene/chlorocarbon solution. The polymer is then isolated by precipitation into a polymer non-solvent and filtered. The Diebel process dissolves the polymer then recovers it by precipitation. Although such reprecipitation procedures are effective in removing occluded impurities, they are ineffective in removing hydrolyzable aliphatic halogen.
Accordingly, it is the primary object of the present invention to provide a method for removing or reducing undesirable aliphatic organic bromine impurities from brominated polystyrene.
Another object is to provide a method for removing occluded inorganic halide impurities from the polymer.
Yet another object is to provide a method for improving the thermal stability and reducing the corrosive nature of brominated polystyrenes.
A still further object of the present invention is to purify the brominated styrenic homo- or co-polymers such as brominated polystyrene without the need for first dissolving the polymer.